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LED bias indicator
LED bias indicator
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Old 29th December 2017, 11:15 AM   #11
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by denny9167 View Post
I found this circuit online that might work itís a window comparator, Iím not sure it would work within a window of less than .2V or so but might be worth looking at.
There are a couple of things a bit crappy about the circuit of post # 8:
1. The op-amps run on a single-sided supply of 5 V, but they are not guaranteed to work below 20 V difference between the positive and the negative supplies.

2. There is nothing except leakage current pulling down the cathodes of the 1N4006's.

It should be more reliable when you change the op-amps into TLC271 or so and add a 100 kohm pull-down resistor after the diodes.

Actually that won't do either, because the output of the TLC271 doesn't go high enough.
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Old 29th December 2017, 11:49 AM   #12
denny9167 is offline denny9167  United States
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Here is a video demonstrating its use.

YouTube

If we are referencing a window of less than a volt than a dual supply
Opamp is needed? Which is the reference voltage of the Curcio Bias circuit.
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Old 29th December 2017, 12:09 PM   #13
MarcelvdG is offline MarcelvdG  Netherlands
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You need an op-amp that works on the supply that you use, and that can lift its output high enough for the 4049 to reliably see it as a high level. With an LM741 it may or may not work, depending on luck.

By the way, you could make an even simpler circuit with an LM393. Connect both outputs, the anode of the LED and a resistor to the supply together, connect the cathode of the LED to ground and connect the inputs as they are connected in the circuit of post # 8, but with the + and - inputs reversed.
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Old 29th December 2017, 12:16 PM   #14
Elvee is offline Elvee  Belgium
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Here, a three LED version of the circuit is discussed:
A simple PushPull balance LED indicator?
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Old 29th December 2017, 12:43 PM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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In post 16, I meant a circuit like the attached sketch. The LM393's inputs have to be reversed compared to those of the LM741s in post 8. The open-collector outputs of the LM393 short the LED when the voltage is out of range.
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Old 29th December 2017, 09:18 PM   #16
denny9167 is offline denny9167  United States
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Worked this up in a iPad app, looks to work ok.
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Old 29th December 2017, 10:06 PM   #17
MarcelvdG is offline MarcelvdG  Netherlands
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I don't get it. You run your op-amps on a single-sided +5 V supply, and still they manage to produce -5 V at their outputs?

If you want to see whether the circuit is sensitive to board leakage, put a 1 Gohm resistor between the input of the 4049 and the +5 V. Is the reverse current of the diodes then still large enough to pull down the inverter's input?

Last edited by MarcelvdG; 29th December 2017 at 10:31 PM.
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Old 30th December 2017, 03:17 PM   #18
denny9167 is offline denny9167  United States
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Apologies, I forgot to set min. Voltage to 0V. It still works.
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Old 30th December 2017, 07:51 PM   #19
MarcelvdG is offline MarcelvdG  Netherlands
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Including the 1 Gohm resistor? Then apparently the diode's reverse leakage is high enough. Was that the reason for choosing 1N4006 diodes rather than small diodes such as 1N4148?

Regarding the op-amp, I think you are right. The LM741 is only specified to work from 20 V to 36 V or 44 V (or +/- 10 V to +/- 18 V or +/- 22 V with split supplies), but when you look at the internal schematic, you see that they should do something on any supply exceeding about five junction forward voltages. The only effect of a low supply voltage will be much reduced performance, because the biasing current through R5 will get much smaller, but that won't matter for a LED bias indicator.

If you model the op-amp as a high-gain controlled source with limits at 0 V and +5 V, that's a bit optimistic, because the LM741's emitter follower output stage will stop working somewhere between 0.6 V and 1 V below the supply. Nonetheless, 5 V - 1 V - 0.5 V or so (for the 1N4006) is still high enough to be seen as a high level by the 4049.
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Old 30th December 2017, 08:40 PM   #20
denny9167 is offline denny9167  United States
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I’m going to play with it somemore, and I believe I’m going to model 12V for the opamp using a dropping resistor and Zener for the inverter, as well as doing some more work with it in B2 Spice, the iOS app I used on my iPad I’m not sure
How good it is. I’ll eventually do some breadboarding with this as well.
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